A giant ring laser, sensitive to the spin of the planet and the twistof earthquakes, will open new windows in earth science

By Eric Hand

LORD OF THE RINGS

The aluminum hatches are the only clue to what lies beneath. Buried amid the corn and wheat fields of Fürstenfeldbruck, a sleepy mon- astery village 20 kilometers from Munich, Germany, is an inverted pyramidofconcrete, steelpipes, and precisionsensors, asdeepasathree- story building. Last month, when
lasers began coursing around
the edges of the tetrahedron,
Rotational Motions in Seismology (ROMY), as it is called, began
its reign as the most sophisticated ring laser in the world, capable of sensing how Earth itself
twists and turns.

“It’s a structure that has
never been built before,” says
Heiner Igel, a seismologist at
Ludwig Maximilian University
in Munich and the principal investigator for the €2.5 million
machine. “It’s something so special.” What makes it singular is
the finesse needed to keep the
lasers stable and to detect tiny
changes in their wavelengths.

The speed and pace of thosemeasurements promise to add an incre-ment of precision to GPS navigation, andROMY may even be able to detect a subtleeffect predicted by Albert Einstein’s theoryof general relativity: the drag of the rotatingplanet on nearby spacetime, like a spoonturned in a pot of honey. ROMY also will besensitive to the weak rotations that accom-pany earthquakes, long-ignored motionsthat contain clues to the interior structureof Earth. By showing the value of record-ing those motions, ROMY could pave theway for miniature sensors that could helpoil and gas prospectors and even planetaryscientists who want to listen for tremors onthe moon and Mars.

RING LASERS ARE EXQUISITE rotation sen-sors thanks to an effect that French physi-cist Georges Sagnac demonstrated in 1913.

He split light into two beams that traveled
in opposite directions around the mirrored
perimeter of a spinning tabletop. When he
recombined the light, he saw interference
“fringes”—dark and bright bands indicating
that the light waves in the two beams were
out of phase. The beam moving in the direction of the spin had traveled slightly farther
than its counterpart, causing the phase shift.

In the decades since, scientists put theSagnac effect to work to track rotations.

The principle underpins the laser and
fiber optic gyroscopes that replaced finicky
mechanical gyros in the 1970s and are now
standard for navigation. The rotations they
measure, like the turns and dives of a fighter
jet, are fast and large. The idea of building
a larger, more sensitive ring laser for geodesy—measuring Earth itself—didn’t come
around until the 1990s, when nearly perfect
mirrors became available.

One of the first such lasers
was C-II, a ring laser in the
shape of a square with 1-meter
arms, built in New Zealand in
the mid-1990s and housed in a
disused World War II bunker,
where temperatures are stable.
Whereas Sagnac shone light
into his experiment from an
external source, the C-II’s ring
itself generated laser beams, its
cavities filled with a lasing medium of neon and helium gas.
As before, a rotation lengthened one light path, but the effect on C-II was to stretch the
wavelength of the laser resonating along that path, like the
coils in a stretched spring. For
the beam running in the opposite direction, the path and
wavelength were squeezed.

When the beams were interfered, their
slightly clashing wavelengths caused the
optical equivalent of the pulsing beats
that piano tuners try to eliminate as they
strike a note and a tuning fork at the same
time. “You have beats because you’re out
of tune,” Igel says. The beat frequency is a
direct measure of the rotation that causes
it, and C-II was able to measure Earth’s rotation rate to one part in a million.

C-II also launched the career of Ulrich

ROM Y’s concrete base is visible in 2016 during construction of the ring laser.